Preheat oven to 375° In a covered saucepan. Cook green beans in a small amount of boiling water for 10 to 15 minutes or until crisp-tender; drain and set aside. Meanwhile, for white sauce, melt butter In a medium saucepan. Stir in flour, dry dressing mix, and white pepper until combined. Stir in milk. Cook and stir over medium heat until thickened and bubbly; remove from heat. Coat an unheated medium nonstick skillet with nonstick cooking spray. Preheat over medium heat. Add onion and garlic; cook for 2 to 3 minutes or until tender. Remove half of the onion mixture; set aside. Add mushrooms to skillet and cook about 5 minutes or until tender. In a 1-1/2-quart casserole, combine mushroom mixture, beans, and white sauce. In a small bowl, stir together reserved onion mixture and bread crumbs. Sprinkle bread crumb mixture over bean mixture in casserole. Bake, uncovered, for 25 to 30 minutes or until heated through.